The water fraction and oil fraction will typically be determined by near infrared (NIR) absorption, for example by means of a differential optical absorption spectrometer (DOAS) in which the attenuation of radiation at a wavelength of an absorption band characteristic of one component of the mixture is compared with the absorption of a reference wavelength in order to determine the proportion of the relevant component in the mixture. Clearly it is desirable to be able to determine the water cut with a high degree of accuracy. For example, at a high water fraction, a small error in the value of the water fraction will lead to a considerable error in the fraction of oil.
Conventional DOAS techniques employ an operating wavelength λm at the centre of the absorption band of a specific molecular constituent of interest, and a non-resonantly absorbing reference wavelength λr. It may be shown that the molecular concentration Nm is related to the ratio of the transmitted optical power P at the two wavelengths by the following equation:
            P      ⁡              (                  d          ,                      λ            m                          )                    P      ⁡              (                  d          ,                      λ            r                          )              =      exp    ⁡          (                        -                      N            m                          ·                              σ            m                    ⁡                      (                          λ              m                        )                          ·        d            )      from which on can obtain:
                              ⇒                      N            m                          =                              1                                                            σ                  m                                ⁡                                  (                                      λ                    m                                    )                                            ·              d                                ⁢                      ln            ⁡                          (                                                P                  ⁡                                      (                                          d                      ,                                              λ                        r                                                              )                                                                    P                  ⁡                                      (                                          d                      ,                                              λ                        m                                                              )                                                              )                                                          (        1        )            in which σm is the molecular absorption cross-section, and                d is the thickness of the material sample.        
One example of a near infrared sensor is disclosed in U.S. Pat. No. 6,292,756 which describes a narrow band infrared water fraction meter in which the infrared radiation is substantially transmitted through the hydrocarbon phase and absorbed by the water phase so that the attenuation of the radiation will give an indication of the water fraction of the mixture.
However, it is still difficult with such a system to obtain an accurate indication of the water fraction of the oil and water mixture. This is largely due to the fact that stream of fluid passing the spectrometer probe will, in general, have a number of spectrally broadband variations in the optical transmission. This is due predominantly to the Mie and Rayleigh scatter from fluid borne particulates such as sand, fine bubbles and emulsions that are likely to be present as random, time dependent quantities with the result that there will be a relatively high background noise to the spectroscopic measurement. Also, the background noise will not be constant but will vary rapidly with time as bubbles, solid matter and the like move past the probe in the flow of the liquid.